Polycythemia vera (PV) is a hematopoietic stem cell disorder characterized by the increased production of red cells, white cells and platelets and complicated by thrombotic and hemorrhagic events, extramedullary hematopoiesis, and transformation to myelofibrosis or acute leukemia (AML), albeit at variable frequencies (FIG. 1) (Spivak, 2002), and many of these clinical features are shared in common with its companion myeloproliferative disorder, primary myelofibrosis (PMF). PV is unique since with phlebotomy alone its natural history can be measured in decades. However, not all PV patients enjoy substantial longevity or freedom from significant complications but, in contrast to PMF, no satisfactory clinical criteria exist for stratification with respect to the risk of disease transformation, and usually no cytogenetic or molecular markers predictive of disease transformation are present before the event (Gaidano et al., 1997). Although JAK2 V617F is expressed in both PV and PMF, these are stem cell disorders and hematopoietic stem cells are not dependent on this tyrosine kinase for either survival or proliferation (Spivak, 2010), nor has the extent of JAK2 V617F expression been useful for prognostic purposes (Barbui et al., 2011). Bone marrow transplantation is the only curative therapy for PV (Kerbauy et al., 2007) and pegylated interferon is the only drug that can induce a molecular remission, although not in all patients (Kiladjian et al., 2008). Both have significant toxicities, while all chemotherapeutic drugs employed to suppress marrow and extramedullary hematopoiesis as supportive therapy can increase the rate of leukemic transformation ten-fold (Najean and Rain, 1997; Berk et al., 1981).
For many malignancies, gene expression profiling (GEP) has been a useful approach for risk stratifying cancer patients with a shared clinical or histologic phenotype (Radich et al., 2006), and for developing predictors of disease behavior irrespective of the clinical phenotype (Lenz et al., 2008).